The present invention relates to electrical switching circuits supported by synchronized mechanical switches, and more particularly to electrical switching circuits used for power input circuits.
Most electrical appliances are powered by the primary power supply in a building. In North America, the primary power supply is typically an alternating current (AC), 110 volts, 60 cycles per second (HZ) voltage source. The primary power supply sources in other parts of the world vary in voltage and frequency, but all of them are AC voltage sources at voltages higher than 100 volts. Most electrical components cannot operate directly under the primary power supply. It is therefore necessary to use power input circuits such as rectifying circuits or voltage converting circuits to convert the high voltage AC power input into power sources that are suitable for electrical components.
FIGS. 1(a-c) are symbolic diagrams showing the most common prior art power input circuits. For the example shown in FIG. 1(a), electrical circuits (107) are using electrical power supplied through a power socket (103) that is connected to the AC primary power supply (101) in a building, while a power input circuit (105) is placed between the AC primary power supply and the electrical circuits (107). For this example, the power input circuit (105) is a bridge rectifier comprising 4 semiconductor diodes (D1-D4). FIG. 2(a) shows a typical waveform of the voltage between the power input connections (Vp, Vn) of the power input circuits as a function of time (t) in a period (T); in the first half period (T/2), the input voltage is positive and peaks at a voltage Vpp; in the second half period, the input voltage changes sign, and swings to negative peak voltage (Vnn) of about the same amplitude. Many electrical components, such as integrated circuits (IC) or aluminum capacitors can only operate when the voltage does not change sign; otherwise those components would burn out. It is therefore necessary to use rectifying circuits, such as the bridge rectifier (105) in FIG. 1(a), to provide rectified power voltages that do not change sign. FIG. 2(b) shows a typical waveform of the voltage between the output terminals (Vo, Vs) of the bridge rectifier (105) in FIG. 1(a). In the first half period (T/2), the output voltage equals the input voltage minus a voltage drop (Vfb); in the second half period, the output voltage equals the absolute value of the input voltage minus Vfb, as shown in FIG. 2(b). The voltage drop Vfb is caused by the forward biased voltage drop of two semiconductor diodes, and Vfb is typically around 1.4 volts. This voltage drop (Vfb) wastes energy, and the wasted energy turns into heat concentrated on the diodes (D1-D4) in the bridge rectifier (105). Therefore, the diodes used for power input circuits are typically special high power diodes equipped with heat sinks. Those diodes (D1-D4) also need to tolerate reverse bias voltages higher than 200 volts. Such high power diodes are bulky and expensive. It is therefore highly desirable to develop rectifying circuits that do not need to use semiconductor diodes. Many electrical components prefer to operate under stable voltages; the voltage waveform in FIG. 2(b) cannot support those electrical components. The most common prior art solution is to place an input storage capacitor (Ci) between the output terminals (Vo, Vs) of the diode bridge, as shown by the symbolic diagram in FIG. 1(b). FIG. 2(c) shows a typical output waveform of the prior art power input circuit (115) in FIG. 1(b). When the input voltage swings toward Vpp or Vnn, the input voltage source (101) charges the storage capacitor (Ci) to a peak voltage (Vpp-Vfb), and then the voltage starts to drop due to power consumed by electrical circuits (107), as shown in FIG. 2(c). This voltage drop (Vdrp) causes undesirable ripples on the output voltage. The amplitude of Vdrp increases when the supported electrical circuits (107) consume more power, and decreases with a larger storage capacitor (Ci). A typical prior art method to reduce this voltage ripple is to use filters. However, power input filters operating at the frequency of primary power supply are typically bulky and expensive. It is therefore highly desirable to develop input circuits that can reduce Vdrp to achieve better voltage stability. In addition, the prior art power input circuit (115) only provides input current when the voltage of the primary power supply is near peak amplitude, while almost no input currents are provided at other times. It is highly desirable to develop input circuits that utilize power more evenly.
The output voltage of the prior art rectifiers (105, 115) always peaks at (Vpp-Vfb), which is typically higher than 150 volts. Many electrical components cannot tolerate such high voltage. It is therefore a common practice to use voltage converters that provide output voltages at voltage levels proper for common circuits. The most common prior art method is to use a transformer (129) before the rectifier (115) as shown in FIG. 1(c). Transformers typically can achieve better than 95% power efficiency while reducing the amplitude of the power input voltage by a pre-determined factor, but transformers that operate at the frequency of primary power supply are typically heavy and bulky. It is therefore highly desirable to provide high efficiency voltage converters that are light and small.
These problems caused by transformers and semiconductor rectifiers can be reduced by prior art high frequency switching circuits using semiconductor switches, but semiconductor switches introduce other problems.
An ideal electrical switch should have zero impedance when the switch is on and infinite impedance when the switch is off. It can conduct infinite current while on, and tolerate infinite voltage while off. An ideal electrical switch also requires zero time to switch between different states, and consumes no power.
There are no ideal switches in practice. The switches used in electrical switching circuits are typically semiconductor devices such as field effect transistors or bipolar transistors. Semiconductor devices can switch between a high conductive state and high resistive state with excellent timing control. They are fast, accurate, cost-efficient, reliable, and small. For low voltage, low power applications, semiconductor switches are very close to ideal switches. However, semiconductor switches typically have relatively low breakdown voltages, making them less useful in supporting high voltage applications. Semiconductor switches operating at voltages higher than 12 volts require special manufacturing processes, and those high voltage semiconductor switches are typically slower, less accurate, and more expensive. Semiconductor devices conduct electrical current through semiconductors, so that the on-resistance of semiconductor devices is typically larger than that of direct connections between electrical conductors. This on-resistance of semiconductor devices is typically not an issue for low power applications, but can be a significant limitation for high power applications.
A mechanical switch is defined as an electrical switch that comprises a movable conductor that connects or disconnects its terminals according to the positions of the movable conductor, and that the electrical connections made by the mechanical switch are formed by direct contact between electrical conductors. The on-impedance of a mechanical switch can be very close to that of an ideal switch because electrical connections are formed by direct contact between electrical conductors. Mechanical switches typically can tolerate high voltages because conductors are physically separated in the off state. Due to their near perfect on/off impedances, mechanical switches are widely used for high voltage and/or high power circuits. However, switching a mechanical switch requires changing the position of a movable conductor, which is typically much slower compared to semiconductor switches. It is also difficult to synchronize the motion of many movable structures in different mechanical switches. Therefore, mechanical switches are typically considered useless for synchronized switching circuits that require accurate timing control at high frequencies.
It is therefore highly desirable to develop switches that have excellent on/off impedances like mechanical switches, and also support accurate timing control useful for synchronized high frequency switching circuits.